Hot-dip galvanization process removes metal oxide scales and rust which form on the surface of the metals, particularly ferrous metals like steel, and coats the metal surface with a protective layer of zinc. The metal is typically passed through an acid bath referred to as pickle liquor during such a process. Frequently, the pickle liquor is dangerous and aggressive solution of hydrochloric or sulfuric acid. As the metal is treated, metallic salts are produced in the corrosive acid bath. However, the corrosive action also occurs on the unoxidized or descaled portions of the metal being pickled, which is an undesirable consequence of the pickling process as the metal so removed is wasted. Sometimes an inhibitor such as thiourea is included in the bath, which somewhat inhibits the corrosion of the de-rusted and un-scaled metal while having little effect on the descaling action. In order to prevent acid evaporation, several additives are used as well. The formation of the metallic salts reduces the concentration of the pickle liquor acid and, as will be discussed later, also undesirably enhances the strong corrosive effect on the descaled metal. Hence, the metallic salts must be removed from the pickle liquor, and fresh acid added thereto. Spent pickle liquor which is relatively high in metallic salts and low in acid is thus typically bled off from the pickle liquor bath. The unused acid in the spent liquor is, therefore, lost unless recovered. Disposal of the spent liquor containing unused acid is difficult both due to the huge volume of the spent pickle liquor and due to the low pH. The European and U.S. regulations on dumping of low pH materials are becoming increasingly strict, and the material generated from the pickling process normally requires neutralization before landfilling. Another problem is posed by aggressive metal consumption in the pickling liquor, which leads to the formation of an uneven metal surface; the uneven surface necessitates a greater thickness of the zinc coating and increases the zinc consumption. Thus, the currently existing hot dip galvanization process is economically quite expensive and environmentally dangerous.
A number of processes have been developed for regenerating the acids and recycling the metallic compounds in pickling processes, particularly wherein the pickling acid comprises hydrochloric acid and the compounds comprise iron oxides. However, these processes require careful monitoring and substantial energy inputs, which prohibitively increases the costs of the pickling process with a recovery system. In addition, when using hydrochloric or sulfuric acid, the environmental pollution is always prominent, and there is need to capture and neutralize hazardous fumes, which increases the energy requirements and the costs still more.
U.S. Pat. No. 8,603,420 discloses a technology including the acid regeneration and recycling, but it relates to hydrochloric acid with all its mentioned drawbacks. It is known that pickling by phosphoric acid is a process without the acid evaporation problems and without strong consumption of the clean metal surface. Phosphoric acid is used to remove rust and corrosion products when pitting and rerusting must be minimized. Phosphoric acid is not as corrosive in concentrated form as sulfuric acid or nitric acid, and it does not fume like hydrochloric, nitric or hydrofluoric acids, and it will not attack stainless steel as does hydrochloric or sulfuric acid. However, the high price limits the use of phosphoric acid in this industry, and the lower pickling rate still more restricts the use of phosphoric acid in large operations. Therefore it is essential to regenerate and to recycle the phosphoric acid in order to decrease the cost of its use in the hot-dip galvanization. U.S. Pat. No. 1,279,331, U.S. Pat. No. 2,558,445 and U.S. Pat. No. 3,438,799 describe phosphoric acid as an additive to sulfuric or hydrochloric acid to prevent iron or steel from subsequent rusting. U.S. Pat. No. 2,087,230 describes regeneration of phosphoric acid in pickling technology using hydrochloric or sulfuric acid for removing ferrous salts, but the method is complex and expensive, and not too reliable. U.S. Pat. No. 1,992,045 provides a rust cleaning or preventing solution based on phosphoric acid with additives of sulfonated vegetable oil and ethyl alcohol. U.S. Pat. No. 2,806,000 describes a stainless steel cleaning solution containing phosphoric acid, minor amounts of ferric ion as a corrosion inhibitor, and agents like oxalic acid or sodium bisulfate to improve the cleaning properties, but discarding the spent pickle liquor after its use makes the process uneconomical. U.S. Pat. No. 2,975,029 describes the use of cation exchange resins in regenerating the pickling acids, including hydrochloric, sulfuric, phosphoric, hydrofluoric, and nitric. CN 102925883 and CN 102021591 provide acidic pickle liquor and degreaser compositions based on phosphoric acid, but without solving the problems associated with disposing the spent compositions. U.S. Pat. No. 4,749,455 provides a method of treating an aqueous phosphoric acid solution with oxalic acid. It is an object of the invention to replace the pickling liquor containing hydrochloric or sulfuric acid by a less dangerous pickling liquor, based on phosphoric acid, and further to provide a safe and cost effective way to recycle said phosphoric acid.
It is also an object of the invention to provide a pickling process that does not form dangerous gaseous side products, that reduces excess digestion of the metal being pickled, that allows continuous regeneration of the pickling liquor and finally that allows removal of the ferrous salt from pickle liquor as a marketable product.
It is further an object of the invention to provide a hot dip galvanization process that allows formation of a uniform, thin but effective zinc coating.
It is another object of the invention to provide an environmentally friendly and cost-effective hot dip galvanization process.
Other objects and advantages of present invention will appear as the description proceeds.